Detonating fuse of petn-polyethylacrylate

ABSTRACT

A detonating fuse having an inert flexible core, and wound around said core a strip of explosive composition comprising particulate pentaerythritol tetranitrate incorporated in a polyethylacrylate rubber.

United States Patent 1 3,698,316

Evans 51 Oct. 17, 1972 DETONATING FUSE OF PETN- [56] References Cited Z Z 1 UNITED STATES PATENTS B ackwwd 922,343 5/1909 Schulman ..102/27 R [73] Assignec: E. l. du Pont de Nemours and Com- 2,1 13,004 4/1938 Snelling ..l02/27 R pany, Wilmington, Del. 3,102,833 9/1963 Schulz ..l49/93 X 3,338,764 8/1967 Evans ..l02/27 R [22] Filed: Dec. 18, 1970 [21] Appl 99,474 Primary Examiner-Verlin R. Pendegrass Attorney-James J. Flynn 57 ABSTRACT 52 vs. C: ..102/2l;21b49;33) A detonating fuse having an inert flexible core, and 511 m. c wound around Said core a Strip of explosive composi [58] Field of Search ..102/27;149/93 tion comprising particulate pentaerythritol tetranitrate incorporated in a polyethylacrylate rubber.

14 Claims, 1 Drawing Figure PATENTED 3.698.316

INVENTOR WILLIAM L. EVANS ATTORNEY BACKGROUND OF THE INVENTION This invention relates to a novel detonating fuse.

Fuses are devices used to transmit energy in the form of fire or explosion. Those that transmit energy in the form of an explosion are termed detonating fuses. Detonating fuses are generally round, flexible cords having a core of high explosive. The explosive core is covered with various materials such as textiles or plastics to confine and to protect it from damage caused by physical abuses or exposure to extreme temperatures, water, oil, or other elements, yet provide such essential features as, for example, tensile strength and flexibility. Detonating fuses are used to initiate charges of high explosives by means of the exploding core, to signal, in the destruction or shaping of materials, or other functions where such explosive energy can be used. Such detonating fuses are initiated by blasting caps or suitable boosters. Primacord is a well-known detonating fuse that contains a high explosive core of pentaerythritol tetranitrate and a textile or plastic covering, i.e., a sheath. Detonating fuses are often lacking in tensile strength and become inflexible and stiffen when used at low temperatures unless extraordinary measures are provided such as additional wrapping. Furthermore, the explosive loading, i.e., grains per lineal foot of fuse, of most detonating fuses cannot be readily and simply adjusted during manufacture or use to supply just the necessary energy required. The present invention provides a detonating fuse that is flexible even under low temperature conditions, has high tensile strength, is simple to manufacture, and is inexpensive to fabricate.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a detonating fuse having an inert core and helically wound around the core a strip of particulate explosive in a polymeric binder. The detonating fuse comprises an elongated core of inert flexible nonexplosive material, e.g., cordage, and helically wound around said core a continuous strip of flexible explosive composition comprising particulate pentaerythritol tetranitrate (PETN) of a particle size whose average major dimension does not exceed about 100 microns, preferably having a size distribution of which about 70% is less than 10 micron diameter, about 25% less than 5 micron diameter, and about 5% less than about 3 micron diameter, incorporated in a binder of a polyethylacrylate rubber. The explosive in the polymeric binder wound around the inert core is in the form of a thin strip or ribbon. Generally, the strip has a thickness of about from 0.005 to 0.125 inch, and usually is no greater than about 0.1 inch. The width of the strip can vary but for most purposes and ease of handling, it is usually between about from 1% to 1 inch. The detonating fuse having a strip of explosive composition wound around the inert core preferably, but not necessarily, contains a sheath. The sheath can cover the strip of explosive material and core to protect them from water, oil and other elements, and as a safeguard against physical abuses. The PETN is incorporated in a binder of a polyethylacrylate rubber such as copolymers of ethylacrylate with acrylonitrile or terpolymers of ethylacrylate with acrylonitrile and styrene, but preferably the binder is polyethylacrylate itself. The detonating fuse is flexible, and can be adjusted to give the desired explosive loading by, for example, simply varying the number of wraps of PETN- binder, i.e., flexible explosive composition, around the inert core.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates a perspective view, partially in section, of the detonating fuse of this invention.

PREFERRED EMBODIMENTS OF THE INVENTION Reference is now made to the drawing illustrating a preferred embodiment of the invention wherein detonating fuse 1 comprises an inert core 2, preferably made of cordage, around which is helically wrapped a continuous strip of flexible explosive composition 3 containing PETN and a polyethylacrylate binder. Sheath 4 covers explosive composition 3 and inert core 2.

Particulate PETN, incorporated in a polyethylacrylate rubber binder, is the cap-sensitive high explosive used in the detonating fuse of the present invention as the source of energy. The particle size of the PETN in the binder is an important consideration in making the fuse and governs, among other things, to some extent, the diameter or thickness of the strip of flexible explosive composition. The particulate PETN has a particle size whose average major dimension does not exceed about 100, and usually not more than 40, microns. Preferably, the PETN has a particle size distribution of which about is less than 10 micron diameter, about 25% less than 5 micron diameter and about 5% less than 3 micron diameter. Such superfine PETN can be prepared by dissolving particulate PETN, e.g. capgrade, in warm (90 F.) acetone to form about a 20% solution. The resulting solution is sprayed into about four times its weight of water and the water is agitated during admixture of the PETN-acetone solution. As mentioned above, the particulate PETN is incorporated in a polyethylacrylate rubber binder and shaped by suitable means, such as extrusion, into a thin ribbon. The PETN used in the detonating core comprises small crystals many of which form agglomerates having small holes, i.e. microholes. Some of the microholes are true cavities which lie at all depths throughout the particles whose average major dimension does not exceed about 100 microns. The amount of PETN used in the flexible explosive composition that is wound around the core is about from 60 to by weight. If substantially greater amounts of PETN are used, the compositions lack the desired degree of cohesiveness, whereas lesser amounts of PETN results in products which have unreliable detonation characteristics. Preferably, it has been found that when about from 70 to 80%, and most preferably about 80%, of the flexible explosive composition is PETN, explosive characteristics and physical properties are optimal.

The polymeric binder in which the particulate PETN is incorporated is a polyethylacrylate rubber having a hardness on the Shore A Durometer hardness scale of about from 20 to 70. The Shore method of measuring hardness consists of measuring the penetration of a truncated, conical indenter under the force of a spring. The scale is arbitrary, from (infinitely soft) to 100 (bone hard). The polyethylacrylate rubber constitutes, by weight, about from to 40, and preferably about from to 35, percent of the flexible explosive composition.

The polyethylacrylate rubber used can comprise, for example, polyethylacrylate or copolymers or terpolymers of ethyl acrylate with other substituted olefins which yield copolymers or terpolymers having the requisite Shore hardness. Such substituted olefins include, for example, styrene, acrylic nitriles, e.g., acrylonitrile, methacrylonitrile, ethacrylonitrile, abutyl acrylontrile, a-phenyl acrylonitrile, achloroacrylonitrile and a-methoxymethacrylonitrile, and acrylic acids, e.g., acrylic acid, methacrylic acid, ethacrylic acid and a-chloro acrylic acid. Preferably, the polyethylacrylate rubber binders in which the particulate PETN is incorporated are polyethylacrylate (Shore hardness about copolymers of 95 to 97% by weight ethylacrylate and 3 to 5% by weight acrylonitrile (Shore hardness about terpolymers of 68 to 72% by weight ethylacrylate, to 27% by weight styrene, and 2 to 5% by weight acrylonitrile (Shore hardness about 65), and mixtures thereof. The polyethylacrylate rubbers can be incorporated with the PETN to form a flexible explosive composition by first dispersing a polyethylacrylate in a liquid carrier which is not a solvent for the polyethylacrylate or the PETN and which, subsequently, can be removed by evaporation. The preferred carrier is water. Thus the preferred from of polyethylacrylates in which PETN is incorporated is a latex containing about from 35 to 60% solids dispersed in an aqueous medium. Such latexes are advantageous because no costly or hazardous organic solvents are required to be used to form a continuous elongated strip of flexible explosive composition. During mixing of the PETN and polyethylacrylate rubbers the mixtures may be heated to slowly remove the water or other carrier. Representative polyethylacrylate rubber latexes are commercially available and referred to as Hycar 2600 X 84 (terpolymer of, by weight, 68 to 72% ethylacrylate, 24 to 27% styrene and 2 to 5% acrylonitrile), Hycar 2601 (polyethylacrylate), and Hycar 2671 (copolymer of, by weight, 95 to 97% ethylacrylate and 3 to 5% acrylonitrile), of which Hycar 2601 is especially preferred. Hycar latexes are products of the B. F. Goodrich Company.

The spacing between wraps of each strip of flexible explosive composition around the inert core can be adjusted to provide a predetermined explosive loading, i.e. grains per lineal foot of fuse. The strip of flexible explosive that is around the inert core can overlap or abut but generally the strip of explosive is at such an angle with the inert core that there is space between each wrap, as shown in the drawing.

Any plasticizer for the polyethylacrylate rubber can be used. The amount of plasticizer incorporated in the polyethylacrylate rubber can vary within wide limits but usually not more than, by weight, 20%, and preferably less than 10% plasticizer is used. Generally, the amount of plasticizer is at least about 2%. Plasticizers which are particularly suitable for use either alone or in combination with the polyethylacrylate rubbers and PETN are esters and polyesters such as, for example, triethyl citrate, tributyl citrate, acetyltriethyl citrate, acetyltributyl citrate, triethyleneglycol di-Z-ethylhexoate, dibutyl phthalate, diisodecyl adipate, diisooctyl adipate, di-2-ethylhexyl adipate, dioctyl adipate, butyl epoxy stearate and di(butoxyethoxyethyDformal. Preferred plasticizers include di- 2-ethylhexyl adipate, dibutyl phthalate and triethyleneglycol di-2-ethylhexoate.

The core of the detonating fuse is elongated and can be made of any flexible material. The material comprising the core is inert to the explosive composition that is helically wrapped therearound. Although the inert core generally has a round cross-sectional area, usually about 0.10 to 0.15 inch thick, it can have different cross-sectional configurations, for example, rectangular. Many materials can be used to form the inert core in the detonating fuse of the present invention such as plastics and cordage. Representative inert cores include those made of nylon yarn, polyester yarn, cordage such as sisal rope, cord, twine, wire and the like.

Preferably, but not necessarily, the detonating fuse is provided with a sheath. The sheath can be in the form of a continuous cover formed by the extrusion of a suitable plastic such as polyethylene or polypropylene around the detonating fuse, or the sheath can be made of a textile material. The sheath can have an open design such as those formed by the process of weaving or braiding and can cover a selected portion of the detonating fuse or, if desired, the entire fuse. For example, the sheath can be in the form of a continuous strip of the same size and shape as the explosive composition which it is wrapped around and secured to adhesively. Preferably, the sheath is made of plastic and covers the entire detonating fuse, that is to say both inert core and helically wrapped explosive, as illustrated in the drawing.

For a clearer understanding of the invention the following specific examples are given. These examples serve to illustrate preferred embodiments of the invention and they are not to be construed as limiting the underlying principles and scope of the invention.

EXAMPLE 1 The detonating fuse was prepared by mixing in a jacketed, kneading-type mixer parts of dry particulate pentaerythritoltetranitrate (PETN) of a size distribution such that about 70% is less than 10 microns diameter, 25% less than 5 microns diameter, and 5% less than 3 microns diameter, 17 parts (dry basis) of the polymer latex of polyethylacrylate (Hycar 2601 and 3 parts of the plasticizer di-Z-ethylhexyladipate. The ingredients were mixed with an agitator operating at 5 to 15 r.p.m. under vacuum to remove substantially all the water. The absolute pressure in the mixer was 10 to 30 mm. of mercury, and hot to F.) water was circulated through the mixer jacket. The resultant product of particulate PETN in a plasticized polyethylacrylate matrix was then extruded at temperatures of 140 to 180 F. as a cylindrical cord which was subsequently rolled to a ribbon having a thickness of 0.015 inch and cut to a width of A: inch. The explosive loading of this ribbon was 7 to 8 grains PETN/lineal foot of ribbon. This ribbon was centered along inch wide paper tape adhesively coated on both sides, and a inch wide polyethylene (PE) tape adhesively coated on one side was then centered along the ribbon with the adhesive face of the PE applied to the ribbon, whereby the ribbon was thus sandwiched between the paper and PE tapes. This sandwich was then spirally wrapped around sisal cordage (with the paper tape facing inwardly) in such a manner as to leave a V4 inch space between adjacent turns of the wrapping. The assembly was immersed in hot tap water for several minutes to shrink the PE. The resulting detonating fuse has a diameter of about 0.20 inch and an explosive loading of about 9 grains/lineal foot of fuse.

The detonating fuse was tested for initiation by No. 6 blasting caps which are conventional commercial blasting caps. In every case the explosive end of one cap was put perpendicular to the fuse axis (T prime) and over the explosive ribbon. The velocity of detonation was 7,000 m./sec. In quintuplicate tests the fuse was unfailingly initiated by the cap.

Additionally, when the fuse was initiated, the detonation initiated a second section of fuse applied as a clove hitch knot to the first section of fuse. ln quintuplicate tests there were no failures.

A single strand sample of the fuse was inserted in a hole (designed to accept detonating fuse) of a A; lb. HDP-3 high explosive primer. Initiation of the fuse caused detonation of the primer.

EXAMPLE 2 The procedure described above in Example 1 for making the detonating fuse was repeated except that the explosive ribbon was 0.025 inch thick, the inert core was a 5/32 inch cotton rope, and the outer sheath was 0.005 inch thick polyethylene. This fuse was cooled to 40 F. at which temperature fuse samples were tied in simple knots. When the fuse was initiated (at about 70 F.) with a No. 6 blasting cap the detonation traveled through the knot in each of four trials, indicating its receptivity to initiation by blasting caps.

A single strand of the fuse was inserted in a hole ofa Va lb. HDP-3 high explosive primer. The fuse was initiated by a No. 6 blasting cap and detonated the primer.

EXAMPLE 3 The procedure described above in Example 1 was repeated except that 70 parts of dry particulate PETN of a size such that the average major dimensions did not exceed 100 microns, 23 parts (dry basis) of the polymer latex of, by weight, 95 to 97% ethyl acrylate and 3 to 5% acrylonitrile (Hycar 2671) and 7 parts of the plasticizer di-2-ethylhexyl adipate were substituted for the corresponding ingredients. The explosive composition was rolled to form a ribbon having a thickness of 0.025 inch. The fuse was initiated with a No. 6 blasting cap T primed over the explosive ribbon, and the velocity of detonation of the fuse was about 7,000 metion of the primer.

EXAMPLE 4 The procedure described above in Example 1 was repeated except 17 parts (dry basis) of the terpolymer latex of, by weight, 68 to 72% ethyl acrylate, 24 to 27% styrene and 2 to 5% acrylonitrile (l-lycar 2600 X 84), and 3 parts of the plasticizer dibutyl phthalate were substituted for the corresponding ingredients. The inert core was 5/32 inch diameter solid polyethylene cord. The detonating fuse was initiated by a No. 6 blasting cap T primed over the explosive ribbon.

A single strand sample of the fuse was initiated in a hole of a is pound HDP-3 high explosive primer. The fuse was initiated by a No. 6 blasting cap and detonated the primer.

I claim:

1. A detonating fuse comprising an elongated core of inert nonexplosive material, helically wound around said core a continuous strip of flexible explosive composition comprising particulate pentaerythritol tetranitrate of a particle size whose average major dimension does not exceed about 100 microns incorporated in a polymeric binder of a polyethylacrylate rubber.

2. The article of claim 1 wherein the binder is polyethylacrylate.

3. The article of claim 1 wherein the binder is a copolymer of ethylacrylate and acrylonitrile.

4. The article of claim 1 wherein the binder is a terpolymer of ethylacrylate, acrylonitrile and styrene.

5. The article of claim 1 wherein the strip of explosive has a thickness of about from 0.005 to 0.125 inch.

6. The article of claim 5 containing about from 60 to 85%, by weight, pentaerythritol tetranitrate.

7. The article of claim 5 wherein the width of the strip of explosive is about from $6 to 1 inch.

8. The article of claim 5 having a sheath covering the strip of explosive.

9. The article of claim 8 wherein the sheath is polyethylene.

10. The article of claim 5 wherein the inert core is sisal cordage.

11. The article of claim 5 wherein the binder contains a plasticizer.

12. The article of claim 11 wherein the plasticizer is ethylhexyladipate.

13. The article of claim 5 wherein the pentaerythritol tetranitrate has a particle size distribution of which about is less than 10 micron diameter, about 25% is less than 5 micron diameter, and about 5% is less than 3 micron diameter.

14. The article of claim 13 wherein the binder is polyethylacrylate. 

2. The article of claim 1 wherein the binder is polyethylacrylate.
 3. The article of claim 1 wherein the binder is a copolymer of ethylacrylate and acrylonitrile.
 4. The article of claim 1 wherein the binder is a terpolymer of ethylacrylate, acrylonitrile and styrene.
 5. The article of claim 1 wherein the strip of explosive has a thickness of about from 0.005 to 0.125 inch.
 6. The article of claim 5 containing about from 60 to 85%, by weight, pentaerythritol tetranitrate.
 7. The article of claim 5 wherein the width of the strip of explosive is about from 1/8 to 1 inch.
 8. The article of claim 5 having a sheath covering the strip of explosive.
 9. The article of claim 8 wherein the sheath is polyethylene.
 10. The article of claim 5 wherein the inert core is sisal cordage.
 11. The article of claim 5 wherein the binder contains a plasticizer.
 12. The article of claim 11 wherein the plasticizer is ethylhexyladipate.
 13. The article of claim 5 wherein the penTaerythritol tetranitrate has a particle size distribution of which about 70% is less than 10 micron diameter, about 25% is less than 5 micron diameter, and about 5% is less than 3 micron diameter.
 14. The article of claim 13 wherein the binder is polyethylacrylate. 